My time traveler is tasked with finding information on previous visits to Earth of a potentially hostile alien civ.
What sort of traces might aliens leave that could identify them? I'm thinking some kind of biological or light trail visible to a type of spectrum analyser...this could pinpoint the civ in question.
A sort of who dunnit, before dunnit gets dun...
He's going back in time to investigate suspected places, but I'm after ideas on what sort of traces (e.g. footprints (not like Armstrong's...) he COULD be looking for to ID the civ. Some kind of DNA trail is my best bet at the moment, but I was hoping for something a bit more exotic! He will be using some kind of scanner to survey terrain for traces. There is also a reason for going back in time as the traces will disappear over time.
I'm thinking something along a snail trail, but different and not readily visible. Perhaps time/space footprint that points to an entity