"I'm truly sorry" could easily be "I'm truly sorry that she's missing, or that her family is going through hell, or just a general statement". Again, I can imagine myself saying that, especially if I'd had no sleep for days and 95% of people were calling me guilty - and they could also be on drugs, or withdrawing from them. If you're going through everything they say with a fine tooth comb, then yes, it's a small red flag - but imo, not more than that - especially given their mental states right now. If an intelligent, educated and very thoughtful person - who wasn't exhausted etc. said "I'm sorry, and I had nothing to do with it" - well, then I'd say the flag would be a bit bigger, and more red. MOO
All good points, especially pointing out how messed up these two seem to be (from whatever it may be - drugs, sleep deprivation, etc.) I hope they didn't do anything to Sydney and that the 'sorry' is for how hard it must be for her family. They haven't been charged and even if they are, they are IUPG. Hoping Sydney is found and is safe. Hoping these two come clean or help out.
I do agree with OP that people who apologize for something tend to have guilty consciences and that could be the case.
Sorry so wishy-washy! So many things to think about.