Yeah, my guess would be low coolant in pressure vessel.  Perhaps some tiny leak in hoses or connections.  Of course, very easy fix to add 50/50 coolant....and then observe for a while.
Side note:  I just ordered the aluminum pressure vessel from Van Cafe.  Not that I'm having any issues of late.  But word on the street is that the plastic tank and the plastic cap tend to degrade over time, and reliable parts are getting hard to come by.  Since I plan to keep my Subie 2.2 Vanagon Tintop for another 30 years and do a lot of long distance travel, I figured what the heck, go for it.

    On Tuesday, February 1, 2022, 11:35:45 AM PST, David McNeely <davmcneely40@gmail.com> wrote:  
 
 Had an odd incident this morning.  Driving at about 35 mph, ambient temp 24
F, no wind, temp needle on the LED as usual, having driven about 5 miles
from cold start, temp led started blinking.  When I could get to a safe
spot, I pulled over, let the vehicle cool, and went on to my destination
(another 8 miles) with no reoccurrance.  I then drove home (13 miles),
stopping to buy coolant just in case.  Right now, the van is in my garage,
waiting cool down to check for low coolant or leaks.

Any thoughts?  By the time you read this, and anyone gets back to me, I
will have checked it of course.

David McNeely