May a helicopter fly at night with only strobe lights?
In the US FAR §91.209   Aircraft lights. applies

No person may:
(a) During the period from sunset to sunrise… (1) Operate an aircraft
unless it has lighted position lights;
…
(b) Operate an aircraft that is equipped with an anticollision light
system, unless it has lighted anticollision lights.

There are no exceptions. A couple of thoughts though. It can be pretty dark on the ground and still not be after sunset. I live in the shadow of a mountain and the sun goes down at my house 15 minutes before sunset.
It is possible that they just forgot. Most pilots who have strobes turn them on all the time (an exception would be in clouds or fog) so they would be on.
It is also possible that they were on, but because of the position of the helicopter, you just didn’t see them.
The beacon (referred to in the FAR as anti-collision light system) does not have to be just red. Lots of them are red in back and white in front, so you may have seen just the forward facing light.
This StackExchange Post has more details on lighting.