DTT stands for detriot metro area and defintely includes:
Coleman A Young Municipal (Detroit, MI) (KDET)
Detroit Metro Wayne Co (Detroit, MI) (KDTW)
Grosse Ile Municipal (Detroit - Grosse Ile, MI) (KONZ)
Willow Run (Detroit, MI) (KYIP)
some include ARB and TOL into DTT, just like HPN is included into NYC
YIP is quite active:
http://flightaware.com/live/airport/KYIP