Even if there's no alien technology, reports will show capabilities and limitations of US radars and other methods of detecting foreign objects. And there's likely stuff about US experimental craft in there as well. Even if outdated, that's something that the DOD isn't going to want out there.
Even the best tech has its weakness's and blind spots, for example if memory serves me right modern body armor isnt exactly immune to arrows and bolts. Plus some tech produces weird blind spots by being too good, if you've got a radar that can detect every bird in 100 mile radius you need to filter out birds but now small drones are also most likely filtered out.
I used the crossbow and bow example with body armor for a reason. Brute forcing something with seemingly simple tech can be rather effective. Sure your super fancy computerized lock may have a million combinations and can change its code at random but ive got a blowtorch from the 50s and a hammer. You see what I mean simple tech can exploit more advanced tech if used properly.