Exhaust smoke.

i noticed it when i was opening the door to my shop. Was returning from a car show that was about 15 miles away(so car was warmed up, running normal temp.)I put it in the shop to check trans. fluid(it is a 904). The smoke did not run me out. This is the first time i saw the smoke. When i started it this morning it seemed fine(no smoke) I am used to start up smoke that disipates almost immediatly.