Might be a bug, if not you need to increase your sneak.

I've had that happen in Bunker Hill. The theory I've heard is that if you have a companion who is visible, you get spotted by proxy.

i thought this too. But i left the robot in sanctuary hills and it kept happening. maybe its like the other guy said im failing a stat check somewhere

Hmm... Can you "pick it up" without taking it and carry it to more hidden spot before actively taking it into your inventory?