Injector o-rings and even the injectors themselves can leak and cold weather is when that shows up and best to check that right at cold start. Regulator is on the fuel filter so it's not that, and the fuel sender is far from the engine bay as well, fuel pressure test won't tell you much about a leak.
If it's like the M54 facelift the regulator is vented to air intake boot so there is a small chance that if the pressure regulator diaphragm is leaking that some fuel is coming from there but that is really rare and usually gets bad quickly.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)