I want to use a mercury based golem as a enemy in a murder mystery P&P story. The golem flows through cracks and under doors and envelops its victims until they are dead. Let's assume that the golem loses some mercury when engaged in a struggle, but not enough to leave drops or pools.
What do the victims look like once they are found? Are there any special features that would be a clear hint about the cause of death?
If they're actually drowned there will be mercury in the lungs, because drowning, in general, is the inhalation of a liquid (though the term is occasionally used, loosely, for inhalation of heavier-than-air gasses).
Even if the mercury golem is able to reclaim the mercury that was forced into the victim's lungs, the "losses" will be detectable as traces in the bottoms of alveoli, caught on bronchial cilia, and so forth.
Even if the victims were suffocated without the mercury entering their lungs, any metals on their person will have been attacked by the mercury, which aggressively forms an amalgam with any metal that isn't protected by a fairly heavy oxide coating. Anodized aluminum is protected, but if there's a scratch or dent that penetrates the anodize, the aluminum will be attacked vigorously where at that point. Gold, silver, copper jewelry will take on the appearance of silver (old school trick, back when students were allowed to handle mercury, was to make "silver pennies" by cleaning them well and then dipping them into mercury).
Metals frames for glasses will be attacked where the lacquer coating is worn off (along the temples, first, then where the frame touches cheeks, nose, or brow), a metal wristwatch band (even stainless steel) will be attacked, though less visibly so, as will the case of the watch itself. Keys in a pocket will be coated with amalgam, unless they're aluminum, then they'll break down completely (look for YouTube videos on mercury attacking aluminum).
You should also consider what your mercury golem weighs. Mercury is about 13.5 times as dense as water, meaning a mercury golem the same size as a human would weigh around a tonne and a quarter. Floors can take a lot of distributed weight -- upright freezers can weigh half a tonne when full, and are probably the heaviest thing, per square meter, you'd have in your house; your mercury golem weighs more than twice that, and stands on (again, assuming similar overall shape to a human) less than a quarter the area. You're likely to find structural damage to floors and joists where the golem has walked or stood.
How do the victims look like once they are found?
You can't drown in mercury, because mercury is really dense (13593 kilograms per cubic meter, about 13.6 denser than water) and you would float above it as soon the quantity is significant.
Mercury is also reluctant to wet most surfaces, meaning that it wont get in the lungs because of capillarity like water would do. For the same reason it wouldn't wet the victim body in any way. Unless maybe removing gold from the body, there would be no trace of mercury bathing.
The only way mercury can kill is either by poisoning or by blunt force (100 kg of mercury dropping on you would kill you like any other substance).
Depends on your mechanism of action.
Engulfing: This will result in very similar results as drowning would. But instead of water infiltrating the airways. Petechial hemorrhages might also be present as is common with victims of asphyxiation, along with cyanosis of lips and extremities. If no sufficient mercury is left behind, some mercury might still be detected in the blood as it can make its way to the bloodstream through skin. So you'd find a corpse, looking like your average asphyxiation victim with a suspiciously high mercury content in their blood.
Crushing: Mercury is incredibly dense. Somewhere in the order of 13.5g/cm³. But since it's a liquid the mechanism is a bit different than a solid just standing on you. If you were submerged by a meter of mercury the pressure would be comparable to being 13.5 meters under water. Even in a swimming pool one feels their ears hurting when one dives only 3 meters deep. If your golem is sufficiently large the victim's lungs will likely collapse, or render them unable to expand even if your victim's head is still in the open air.
No matter how, the main mechanism of death is hypoxic of nature, so victims should show signs of asphyxiation and high mercury presence in blood.
Due to the nature of quicksilver victims are likely to have little to no blunt force trauma. The victim will likely show no external signs of strangulation or ligature marks otherwisely found on victims of asphixiation away from any regular source of asphyxiation (like a body of water, or debris capable of crushing someone sufficiently to inhibit chest expansion.)
You will have to find something to keep humans engulfed in your golem down, since humans are a lot less dense than your mercury golem they will float naturally.
Collapsed Lungs
Other answers have pointed out how mercury is dense. If you fell into a pool of mercury you would go splat rather than splash. That means it is difficult for your golem to envelop the victim without them simply floating to the top.
Instead I suggest your golem invade their body by perhaps only enveloping the head and then pumping mercury into the lungs until they stop working.
Of course two lungfuls of mercury weighs about the same as grown man. So the lungs will burst long before they fill up, and the victim will die in a much slower and more gruesome manner than you describe.
