I've had it before where they swap out your rechargeables for others. I had some dodgy ones once so replaced them with brand new which cost an arm and leg as they're special ones only for them to swap them out for someone elses at Service!
Had an analyser back before too with the Infra Red printer sender no working, when it did when it went in. Also had a faulty switch once which I mentioned on service and didn't get rectified, took it back and to be fair they did it FOC.