This is indeed very weird. Unfortunately I don't have any experience with your motherboard but hopefully somebody knows what's going on.
@TommyL BTW, what bmc/bios revision are you running? On SM there is R 3.48 for BMC and R 3.3 for BIOS
Could be a bunch of things. If you stripped it down to bare minimum and it's still not powering on then reset bios and run it with one stick of ram and leave only one fan connected. There is a slim chance it is a failed fan. Just keep trying, it moght be something you missed.
Good luck man. Working as a freelancer can be a real bother when It comes to getting paid sometimes. If you had any connection with the client while you were working for the company it will most probably go smoothly.
Could be bad capacitor chemistry. Failed caps can pop from underneath(older electrolitic ones used to fail like that, they didn't pop fro' above as they should).
Take a closer look and if you can please post a pic for future references.