Chiming in a little late. I've also seen a bunch of Mountain Chickadees in my yard and neighborhood in Littleton, JeffCo., next to Clement Park. Before a couple weeks ago, I had one yard record, now I have up to 3 daily in the yard (yay!). There's also been a flock of Steller's Jays around S Platte Delta area at Chatfield. I saw about 10 (silent group!) a few weeks ago from the Heronry overlook in Douglas County.
To add to the discussion, I looked at eBird to see if there was a discernible difference in the number of reports of Mountain Chickadees and Steller's Jays in the Denver area for Aug-Oct 2019 and 2020. See attached the jpegs. Pretty noticeable influx this year for both species. There's been a pretty good number of Mountain Chickadees across the SE plains recently as well (not shown in my maps, you'll need to explore eBird). There's also been a bunch reported in the OK panhandle this week as well, plus Cassin's Finch, Steller's and Woodhouse's Scrub Jays, even Evening Grosbeak, and a super rare for OK Black-capped Chickadee. This could be a really interesting winter!
As others have said, Pinyon Jays can be highly nomadic in fall and winter when food resources in their home ranges are scarce. There often seems to be a lone bird or a small group that turns up in weird places far from where they usually occur.
That said, I'd encourage everyone who is inclined to use eBird, to continue reporting your birds to help document the apparent unusual movement of some species out of the foothills and mountains and into the plains.
Scott Somershoe
Littleton CO