It could definitely be just old leaves that are beyond their prime and need to be removed.
Does it look anything like this, tho?
Heuchera rust.

Here is a link that might be useful: Heuchera rust

Nope not like that. I'll post a pic tomorrow. They are more blotchy and larger patches of brown.

I've been having brown patches on Caramel Heuchera just in the last 2-3 weeks. They're a new plant for me and only ten months in ground. It's the mature leaves that are affected and they have subsequently shrivelled up and fallen off completely. Could this condition be "normal" considering the unusually cool, wet Spring we've had in Seattle? Many of the plants in my yard seem a little, well, confused this year.

I clipped off the dead/dying leaves, and there are new, so far healthy, baby leaves growing below. So I'm crossing my fingers that nutmeg4061 is correct, and that the plants will recover.

Citronelle is noted for these brown patches - I call them "sunburn" and clip them off. Have noticed this on Caramel just during the past couple of weeks as we've gotten into the awful heat.

I was thinking sunburn, a photo would really help.

Here's a picture of Citronelle and some others earlier this month. You can see the patches on Citronelle. Well, the picture doesn't show up very large on this forum so maybe you can't see it. Citroneele is the chartreuse in the distance. Caramel is the gold colored one- it wasn't showing burned spots earlier in June but is now as it is hotter and drier.