carmellia, I am not sure that I understand just how the leaves are falling off. Here is a picture of the base of a Calamondin leaf. Are you saying that a portion of it remains attached to the stalk of the plant? If the leaves are green and healthy when they fall, the plant is telling you that it is upset over something that you did to it recently - probably over watered.

Yes,the little stem that attaches the leaf to the main stem remains. It appears that the leaf is snipped from its little stem. It happens only on particular stems. It starts at the tip and works its way leaf by leaf down toward the main branches. Six stems are currently affected. The rest of the plant has lost no leaves. That is what makes me think it is bugs concentrated on certain stems.

carmellia, Well!!! I found out that I do not know how and where the Calamondin leaf increases in size and maintains the proportion of its (3) sections. What follows is pure conjecture.
I'd say that we can rule out insect or disease. I can think of no good reason why an insect or pathogen would target a particular tissue on the leaves of a single stem.
Then it must be internal; and "hormone effect" immediately comes to mind. Hormone imbalance can explain abscission of (all) leaves, starting in the apical zone and working downward on a stem. But such separation should take place at the juncture of leaf-stalk and main stem, shouldn't it?
Maybe there is another abscission layer at the base of the leaf blade (similar to the dewlap on a grass leaf). This would cause the leaf-blade to fall away and leave (no pun intended) the petiole attached to the main stem.
That is the best story I can come up with; (OMG 2 prepositions!!) and I am sticking to it until someone comes up with better one. Maybe we should post this on the Botany forum.
"Izz a Puzzlement!"; and I wish to thank you very much for it.

w/o a picture.. its hard to go very far with this ...

how long have you had it ... have you had it thru a winter already????

in the great white north.. at this time of year.. it is dark outside [declination of the sun and all] ... and unless you have it under supplemental light.. i would suggest it might simply be losing excess leafage ... and having it only by a window that cuts UV rays does not help ... [and i hope its not sitting on the window sill]

also ... citrus is a near tropical plant ... and if you have it near a window for the light.. and your furnace kicks the heat way down at night.. it might simply be getting too cold ... not so cold as to kill it.. but to harm some leaves ... like all that news from FL when they get a chill and it harms the orange crop ...

then.. in your z4.. there is the whole lack of ambient humidity in a heated house .... and watering issues.. etc ...

how we figure out which variable is the culprit.. is beyond me ...

BTW.. if the leaf fell off.. the leaf was not healthy ... even if it looked OK ...

good luck

ken