EP:
How do you make the leap from "engines taken away on trucks" to "aliens"?
If you are asking me, I don't. (Unless, of course, you count Chinese and Iranians as "aliens" to Israelis. I imagine to anyone who was capable of traveling across the galaxy, an F110 turbofan would seem hopelessly antique and inefficient. It might be suitable for an alien museum of sorts.
Making contact with persons of another world, or even finding positive proof of such would be a spectacular find. But I think most 'sightings' can be more easily explained by other things. And the mysterious sightings and supposed evidence that remain are too furtive. More like someone was trying to make you think that, but without actually allowing any positive identification.
The immense physics involved in providing transport across light-years of space, only to play hide and seek with humanity afterwards doesn't make sense to me. Either they would stay hidden and observe (ala Star Trek's non-interference policy) or they would reveal themselves (either as hostiles or as friends).
And if their intention was to reveal themselves, it would, to me, seem to be 'the sooner the better.' If as 'friends' hoping to help us, 'sooner' would mean keeping us further from the precipice of self-destruction. If as 'hostiles,' 'sooner' would mean having to contend with less human ability to fight back.
To be sure, the subject inspires possibilities and conjectures and dreams. And I like reading about those possibilities. But reality seems to be more mundane.