Prime probs didn't want to alert any possible snoopers what his plan may or may not have been. And as for the other bots being back and it being too cluttered.....thats a story and plot based opinion.....what if everyone was there but everyone had a solid plot driven purpose in the films one that we could all relate to and for the most part with the less important characters they could just do something that can be edited into scenes using minimal timing but provide substance and further depth to the story as it unfolds. Quite simple a great deal of imagination has been used on these films....sadly this has only been from the visual fx and bot design crews, to whom i applaud their efforts but the attempt to build a rich story and lasting franchise has been approached with a clueless fashion.

And my opinion on the OP subject of this thread is that in one way or another all of the transformers on earth were in that room and it wouldn't surprise me if there are more bots littered about that room

 

The Autobots got Lockdown's ship and ready to leave Earth, but where would they go? Cybertron?

 

^ "Any planet but here" - Epps.