Definitely had water intrusion at some point but I could not locate the source and it was dry at the time of my inspection.
Service panel update install, horrible work, bonding screw loose, never tightened!? Ground wires too short so they twisted 5 together at a time (14 & 12 ga) and ran a 14 ga jumper. No bonding at all. And the coup de grâce, the main ground line was too short to make it to the bus so they just left it off :O/
I've seen what Afjes mentioned about the sealant on many, many ITE (Now Seimens) from the 1980's, but not the bolt-ons.
It's very prominent if the panels are in a remotely damp location. A rep once told me what Afjes siad- The powder is the "protective" grease they use in the wire termination area that dried & stopped protecting.
Actually, one load center in our home is ITE from 1984 and it had the problem. The bus-stabs were dry, but clean and good. Many circuit breaker termination and neutral screws came loose when the grease dried and powdered away, as did the line side screws at the main.
To solve the problem, I cleaned off the ITE "protective" grease and used a zinc-castor oil based brand of anti-oxidant paste. As good practice, I also used it on the bus bars' breaker stab points which also suffered from the white powder. I check it annually and things stayed good.