2001 Buick Lesabre Check charging system message on dash. A

I had my car's engine cleaned. On my way home the dashboard lit up like a Christmas tree (Brake, ABS, Air Bag, Battery) and I got a problem message which said: "Check charging system" this switched with another message which said "EXP 102".
I took it to the garage and the mechanic worked diagnosed it for about 1hr and a half. He figures it could be the alternator, but that it may have been affected by water during the engine cleaning and that MAYBE it will go away after a few days. The alternator will be replaced if the problem persists next week. In the meantime, asside from the dashboard driving me nuts, the car runs perfectly. Any thoughts?