I noticed the same thing on an Apache helicopter at an airshow a week ago. After the engine startup, it appeared. Any help with this would be appreciated. (I didn't know what that thing was, and it is kind of hard to do a search on "What is this Thing?". Bear with me, someone might have already asked this.)
AvionicMech From United Kingdom, joined Jan 2005, 315 posts, RR: 3 Reply 2, posted (8 years 7 months 5 days 9 hours ago) and read 1957 times:
As EMBQA mentioned I think what you are referring to is one of the gear doors. If you look at this picture of another UA 744 you can see that it is the gear door. This is assuming that we are looking at the same thing as you are.