It's the milk in the chocolate caramelizing through the combo of heat and time.  Even at low heat, all you're doing is expanding the time for maillard reaction to take place, which we see quickly at higher temps.  

So for milk chocolate, it may be better to melt and get on with what you're doing rather than leaving it in for long periods of time. If it doesn't matter to you, just expect that over time it will become more and more caramelized over time.

Something to note; while you may not have noticed it because of a minimal change, you'll want to be careful for how long it stays in. As part of caramelizing the milk, some of the acids are also driven off which can change the mouthfeel of the finished chocolate.  Something I've noticed when working with my own formulation of a caramel chocolate purposefully done.

updated by @timwilde: 01/07/17 21:22:16

I'm a new owner of a tempering wheel and a melter and am not sure whether my problem is related to my current setting or the use of the machine. I have noticed that after a few days, the chocolate changes the taste. It becomes a bit more...roasted/caramelized, I guess. Not necessarily bad, but definitely a different flavour from day 1, which does not bode well for big batches of bars. I should add I am talking about milk chocolate here.

My melter is currently set up in my kitchen, which I also use for regular food prep, so smoky/meaty flavours abound. My melter is covered most of the time, either with a plexiglas cover or a metal cover (which leaves about 10% of the pan open to the air).

Sometimes I turn the melter off for the night, sometimes I leave it on low, then reheat the chocolate to 40+ C in the morning and let it temper down to 28-32 C before I use it.

Is the change in taste because I keep reheating/cooling the chocolate, keeping it warm for days, or is it just chocolate absorbing my cooking flavours?

updated by @parole-chocolates: 01/07/17 16:00:59