As far as I know, these pumps are for either of two reasons.
1, there isn't enough slope to carry the waste to the sewer pipe,
2, the city requires sewage to be ground before depositing.
Were it the latter, other houses of the same vintage should have a grinder (sewage ejector) as well - but if it were a new regulation, other houses may be grandfathered in.
There is a distinct possibility the pump was stolen - they run about $1500 a piece. If stolen, the first thing the theif would do is disable the alarm so as not to alert you of his project.
I think you're simply going to have to pump it out - and old sump pump would be an easy way to go. Can't hurt to dump a gallon of bleach down there first.
Keep in mind the toilet paper may jam the pump.
Typically, the cover is screwed down in several places, as well as having mechanical clasps.