In my story, Humans invade one of the portals and set up a base. Before they can solidly their foothold, the aliens attack and 'kill all humans'.
It is very unlikely that such an invasion could have been okayed without at least a very basic recognition and survey.
So I think it's a given that, before the invasion, the Earth forces know what they're going to fight against. They know they have a technological advantage. They have read and upvoted Cognisant's excellent argument.
Therefore, they'd do whatever is in their power to be very, very sure they're not going to lose that advantage. The other side has a whole world, and they're on a war footing.
It becomes crucial to prevent the enemy from using, study, or, worse still, reverse engineer Terran weapons.
In ancient times, when (say) a cannon had to be abandoned in the field, it was either burst or spiked to prevent it from being captured and used. Something of the kind would undoubtedly be done here (or would it?), but much more thoroughly; we don't just want to prevent the enemy from using the equipment (the equipment's complexity would be proof enough for that), we ideally want to leave them nothing they can study.
So provisions to destroy matériel before capture would certainly be in place.
But this only covers foreseeable losses. Equipment might be lost before there is a practical possibility to scuttle it. And it is conceivable that it could be moved elsewhere, to an aliens' Area 51, where it would be examined with great care.
Therefore, I'm sure that some kind of automatic self-destruct would be deemed essential.
So after wiping out all the humans (and assuming what follows hasn't already happened on the Terran commander's orders), the aliens enter what remains of the base, perhaps try breaking into a helicopter...
"By the Elder Gods, Sarge! Have you ever seen anything like this?"
"Less gawking and more technology stealing, soldier! Have you
figured how to start this Gods-cursed contraption? Careful with those
"It's okay, Sarge, we're good-"
ENTER OVERRIDE CODE
"This thing talks!"
UNAUTHORIZED TAMPERING DETECTED. ENTER OVERRIDE CODE.
"What did it say?"
"How the Hells should I know? Do I look like I speak Terran?"
TAMPER BEACON ACTIVATED.
"Hells, isn't red the Terran color for danger? This thing is flashing red!"
"Everyone back! EVERYONE BACK!!!"
52 TAMPERED DEVICES FOUND ON LOCAL NETWORK.
CONTACT WITH HQ NOT ESTABLISHED.
CASE OMEGA ACTIVATED.
I think it quite likely that Case Omega would involve a low-yield, "suitcase" nuclear device triggered by a dead man's switch.
In addition, there would surely be several HE souvenirs hidden in all mobile units. A network of sensors to tell whether there's someone in the cabin; if there is, at least one of the several keypads needs to enter a four-digits code within one minute from the beginning of the beeps.
You could easily mass-produce such booby-traps, and they would be very safe through massive redundancy: in a helicopter you might have, say, six armored sensor-keypad-transmitters, and as many bombs, all self-networking. A bomb would not arm unless networked with at least three sensors and two bombs, and a single OKAY stops it until all networked sensors send a
NO LIFE ABOARD - REARM signal. Damaging the vehicle so much that the transmitters die means there's no vehicle left; finding and securing all the bombs in less than one minute is a losing proposition. Jamming a military short-range frequency-hopping encrypted WiFi inside a metal shell would require such a massive, easily-recognizable-as-such jamming that just trying it would trigger the explosion.