It could be a leaking case joint, it could even be cracked cases. Either will require a full strip down to fix properly. But Triumphs have numerous other potential leak points that are far more likely IMHO...
You’ve got to clean the old oil away if you wanna find the source of the leak. Give it a proper clean down with Jizer / Gunk etc, get it running again and have a good look around it.
Crank case pressure is definitely a good thing to look at first. It’s very easy for someone to fit a solid cam, or similar, by mistake and block off the timed breather.
Then there’s the sump filter area, lots of gaskets and studs there to leak.
Pushrod tube seals are a place where many (most?) Triumphs leak, yes it’s a weak design but the real problem is Triumph subtly changed the design of tubes and seals et al over the years so most bikes today will have a mix n match collection of parts, and most people today do not know what’s right or wrong.
Base gaskets and rocker box gaskets are other favourite leak points.
Any leak above the cases will obviously make its way down and can easily look like a crank case leak.