The leaf drop could still be a reaction to the change in conditions from when it was brought inside, especially if it's the older (lower, interior) leaves shedding. Citrus are sensitive to over-watering, and leaf drop can also be a symptom of that issue, so since they will use less moisture while indoors since growth is minimal, avoid keeping the soil too damp by feeling it about two inches deep (for pots this size) before watering. It should be fairly dry to the touch at that depth before the plants will need water again. When they do need water, drench them thoroughly so excess moisture rains freely out of the bottom drain hole(s); this way, you know the entire soil volume was well-moistened to reduce the likelihood there's a pocket of dry soil within the root ball. (As can happen if waterings are too light, especially after the soil dries out more fully.) Make sure the saucers are promptly emptied after watering so any excess moisture doesn't seep back into the drain holes, which can saturate the lower levels of soil and drown roots.

Citrus appreciate bright light, relatively cool temperatures, and moderate to higher humidity while overwintering indoors. A room humidifier can help if your ambient humidity is low, which it often is in homes in our area in the winter. If you don't have a very sunny window available, you can supplement their light using a plant grow light (there are myriad styles, though anything white that's full-spectrum and ideally intended for growing plants should suffice). You can find more tips on our Growing Dwarf Citrus page.

We see a floor air vent next to the plants, and if it's in use (that is, not closed), then the dry warm air wafting over the foliage might be adding to plant stress. If you don't have an alternative location for the pots that's cooler or not as drafty and can't close the vent, then locating a humidifier near the plants may help to counteract any negative impacts it may have.

You can also inspect the leaves and young stems closely for signs of scale insects or spider mites, two common pests on citrus, as plant stress from their feeding can also lead to leaf drop. While mites are definitely tiny, they will still be visible (more readily if you have a good magnifier handy), so if not seen then they may not be an issue.

Miri