I'm not an engineer on TV and I didn't stay at a Holiday Inn (where's MNBoxster?), but I believe H2O is a byproduct of an engine that is not fully warmed up. Has something to do with meeting emissions and making the engine run smooth when its cold. So that's where the water out the exhaust comes from. If you do not reach normal operating temps before you shut your engine off, the water will not have a chance to evaporate from within the muffler, will collect there, and rust.
10 min may be on the leading edge of a fully warmed up engine. What can you do to prevent this? Take the long way to work and enjoy your Box for an extra 10 min.
Seriously, if you have not experienced a rusted muffler in your other cars in your 10 min travel to work, chances are the Box will not either. Advances in today's metals used in muffleres and coatings such as zinc make it less likey they will rust.