Had almost this exact thing happen with my old Crown Vic a few years back. Went in for a charging issue, sat there three weeks, came out reading way higher than when I dropped it off. First thing I did was dig through my paperwork: registration renewals, inspection records, insurance photos, anything with the mileage written down. That gave me a paper trail to bring to the shop so it wasn't just my word against theirs.
Then I asked the shop directly if they swapped or touched the instrument cluster. Turns out that's the most common cause. On older cars with analog odometers, the mileage lives in the cluster itself, not the computer. If they pulled a used cluster from a donor car to fix an electrical fault, the odometer shows whatever that donor car had on it. Mine had a junkyard cluster with a bunch more miles, and the shop covered the difference by noting it on the repair invoice.
One more thing worth checking before you call: test drives. A car sitting six months while they chase an electrical gremlin can rack up real road miles if they were diagnosing it around town. That's ussually a few hundred at most though, not tens of thousands. If nobody at the shop admits to a cluster swap, ask to see the old part. If they can't produce it, get the mileage discrepancy documented in writing for your records.