Sounds like a bunch of guys who don't know what the fuck they're talking about. If there is anymore damage A supplement will be written. This is not an uncommon practice. Any legit body shop will get it straightened out. Just don't be using a backyard shop. You can take it where ever you want. Only thing I find weird is you were already given a check. Never heard of that especially if there is a lien on the vehicle. You can't really tell the extent of the damage without digging into the truck. Generally an estimate would get written the shop would start their work. Find any additional problems and turn it in to insurance. Thus a supplement is written for additional repairs.
Pretty much what I was told, even if the check was already written. If the body shop finds a problem, they will let the insurance know. He is the neighboring business to where I work and we have him work on all of our trucks, so I am not worried. He wrote the check so the shop could start working on it while my insurance guy starts to hound her insurance to get money.