Agreed. Sounds like water. How often do you perform lube oil analysis. The water should show up in the analysis.
Also, don’t know about your configuration but do you have any lube oil cross tie lines with other engines that the blind mentioned does not isolate. If so, you may want to check the cross tie isolations.
Again – don’t know about your configuration but I know of facilities that ended up with fuel in the lube oil – is that a possibility at your facility.
EGS had this problem and the cause was a valve left open between the new lube oil storage tank and the engine. The system was able to siphon about a quart a minute into the sump when the engine was off. After several days the level was high enough that when the engine was started it forced oil out of the access doors.