Sure it's hard to say, I usually put about 2-3g to clean the chute before a new session and it usually does the trick.

I use a K10 now, but when I used the BV regularly for espresso grind, the retention was fairly low but quite variable depending on the coffee, grind setting and humidity. It was never zero, but sometimes was just a few tenths of a gram. I don't recall retention as high as 2g, but it's been a while. I would guess it averaged .2g-.5g, with occasional higher excursions.

These days I use the BV strictly for vac pot at a couple of notches coarser than the middle drip position. The retention is, for all practical purposes, zero with that coarse of a grind. I think more coffee sticks to the sides of the plastic bin via static than gets retained inside the grinder. If I put 21.5g in the hopper, I get 21.5g of grinds almost every time. When I don't it's 21.4g.

My conclusion is that the powdery consistency of espresso grind, especially at low doses, makes it more likely that you'll get some retention with any grinder, even if you single dose and even with the BV, which probably has the best retention characteristics on the market. Sometimes it adheres to the burrs, burr chamber, chute, etc., and sometimes to a layer of coffee previously deposited, depending on the grinder geometry and other factors. While some of it can stay inside the grinder indefinitely, my sense is that sometimes these grinds fall off on their own accord, perhaps more so with drier ambient air conditions. I've certainly seen grinds fall out of the BV chute several minutes after grinding, and the variability in the retention makes me suspect that sometimes a little old coffee gets mixed in with the new grind. I think in most cases it's not enough to significantly affect taste. I can't imagine that there's any grinder out there that isn't subject to this phenomenon.

FWIW, I single dose with the K10, and it typically retains more grinds on the first use of the day, around .7g, then quite a bit less on subsequent grinds, around .5g or less. I sometimes use a high-power portable shop vac to suck out whatever loose grinds I can from my K10. I used to do this daily, but now I mostly do it when changing coffees or during the weekly detergent backflush. I didn't notice a difference in taste when I stopped vacuuming daily.