Since you don`t use the car often, what happens is the condensation builds up and is not hot enough to evaporate the water so it`s left in the exhaust

 

I've only noticed on the TL, and i didn't see it on my previous car (protege5). Simliar distances for the trips. I drive about 15 minutes to get to work..

but on the good side

 

Back from the dealer they says the rust is normal... and says its only condensation blah blah and the exhaust is made out of stainless steel..

He also checked out other TL's exhaust to see if they had rust or not..

 

They polished them for me but couldn't remove the pits, I meant to take a pic but have been too busy, sorry.