I noticed approx 400 more silvercoins on my account than before, but the report section showed no sign of a transaction.  Could it be a bug giving out people random silvercoins? Or perhaps the Reports section got a bug that refuses to list all the transactions/events?

There are no bugs which are just giving or taking random silver coins (random balance changes need to be coded, they can't be just a bug from other game aspect).

Reports do not list all of the transactions and events, they show only the events which you didn't participate manually (for example, stud fees from a male on the breeding offers, ended auctions, battle results from pets who are standing on arena in auto mode, etc.). If the action requires your direct attention, it won't be listed in logs since you're already aware of it. You can get silver coins which aren't listed in Reports from any direct sale or from a manually chosen battle with bid, for example.