The roughly two-hourly outer suburban service in/out of Euston in the
early hours used to "borrow" the Bletchley-Bedford and/or Watford-St
Albans DMUs if the juice wasn't available, so that's another possible
reason depending on when it was seen.
--
There was a Class 150 in platform 1 at Watford Junction at about 2 o'clock
yesterday afternoon. Not sure if this was the same train though.

Burkey