fly-by-night

Short definition from Wiktionary

Noun

1. 1
   A creature which flies at night; a nocturnal flier or traveler.
2. 2
   One who departs or flees at night in order to avoid creditors, law enforcement etc.
3. 3
   A person or business that appears and/or disappears rapidly, appearing untrustworthy, or giving an impression of transience.
4. 4
   A traveling businessman or tradesman.