That's signifying that at least one of the installed batteries is defective, not taking a charge.
Eneloops are good batteries, that last longer than some of the other brands available.
These
Ladda batteries, available from Ikea, appear to be the same as the Eneloops, with a different name. I've had good results with these, as have many others here on RadioReference.
Amazon has the Eneloops, and a
number of other brands. The EBL batteries have worked well for me, but not always for some other members. The 'charge' function of the HP-2 (as with most Uniden scanners) is not a 'smart charger, that can recognize good batteries, only charge them to the optimum capacity. For Uniden scanners, other than the SDS100 (which has a proprietary battery, not AA), charging is based solely on time, which is set by telling the scanner the capacity of the installed cells. That determines the amount of time the batteries will be charged, and has no ability to recognize a battery that is already charged. It will charge the preset time (based on the capacity you specified), then turn off. You'll get much better, and consistent, results using a 'smart charger' that can recognize how much charge (if any) a battery needs. These chargers also can spot a failing cell, and some models have a 'refresh' setting that in some cases can bring a cell back up to a usable condition. A 'smart' charger could recharge the batteries in your HP-2, and point out which cell(s) need replacement.