I've come across a number of trunked systems, both LTR and Motorola, which do not have a trunked service code -- usually IG or PW. I'm not sure if this is endorsed by the FCC or an oversight, but in any event I wouldn't restrict searching to YG.
Regardless, though, I don't find any bases (let alone repeaters) licensed for 451.9125 within Middlesex, Somerset, or Union counties using the FCC's geo search. Include mobile areas of operation within Middlesex and you get a handful but none jump out as Mandy's or a provider.
One issue with the FCC's geo search is that is relies upon the latitude and longitude data contained within the license, compared against a defined polygon for each county, even for state/county searches. So if some applicant got the wrong coordinates or made a mistake inputting the data, a license which otherwise has the correct street, city, and county data will not appear. The old database interface would look at the county field but that interface has been phased out.
I did a search for a 100 mile radius, centered upon Edison, excluding mobile areas of operation, including only regular authorizations, and including all license statuses (active/expired/etc) and got: zero results. So it rather looks like unlicensed operation unless they were more than a few degrees off with the coordinates.
How did you find 451.9125, close calling it or its input? Does it have idle bursts frequently? Are you able to see a display (if any) on the radio? Sometimes they offer clues to the repeater site or owner.
Jim