So, how did he manage to elude all of the security cameras while carrying a big garment bag (which was probably also pretty heavy)? Simple. He didn't.
The thief tells TMZ that he snuck into the room WHILE Lupita was in there. He knew where the security cameras were in the halls, so he got around those, saw the door was ajar and let himself in. Now this is where it gets really crazy. He took the dress, which was worth an estimated $150,000 (turns out it's not) and threw it off the balcony where someone on the ground was waiting to grab it.
The guy says he also saw her Oscars gift bag, worth a reported $125,000 and left that there, but obviously did make off with the dress - which, as it turns out, is actually made of fake pearls and not really worth anything near what the earlier estimates valued it at.