displays
As the poster above said, even if you've hidden your psionic powers you still go glassy eyed as you concentrate (still have to make a concentration check to avoid AoO etc)
Generally psionic powers have displays (special effects really) that draw attention to the psion. Like spell componets there are a number of them and they have one letter codes to indicate which powers have which sort of displays. The "Psionics should be totally undetectable" crowd in particularly didn't like that in the last version, but it was touted as a balance feature. (depending on how you read the rules there were some funny consiquences: in particular invisibility created a constant ringing as the person was wandering around). In 3.0 you could avoid having displays when you manifested powers by taking feats -and- spending extra power points but there were 5 feats in all and it was prohibitively expensive to hide your powers on a regular basis.
This time around you can try to manifest a power without displays, but only if you make an extra concentration check, flub it and you lose the power.
If I have it right nothing can hide the actual powers effect so hading displays for most powers isn't too useful. Making the check to hide the display for an energy power might get ride of the loud mental shout (an auditory display), but a blast of fire would still shoot from the psion to the target.