newshound presented the following explanation :
I have a Brother HL-2365DW which I guess is rated for SOHO rather than heavy
business use. I'm getting toner spillage leading to (fused) blotchy marks all
over printout, I guess the waste toner collector is full. (it's probably had
ten or more new cartridges). But I can't immediately find any reference to
this sort of problem on the net. I can see that some heavier duty machines
have a removable and replaceable collector but I can't find anything for this
model.

I thought I would ask for tips before trying to dismantle it.

TIA

Usually the waste toner is collected in the cartridge itself, or at
least I have never seen a small or desktop that does not. If they are
smudging, time for a new cartridge.

Inkjet printers use a pad to collect the waste, in the base of the
printer.