What is the significance of "?" in place of the normal Nautilus wireless icon?

From time to time, the normal wireless connection icon changes to a question mark.
Ubuntu 18.04. Wireless adapter is TP-Link TL-WN881ND.

At such times, I haven't noticed and significant difference in connectivity.

I would like to understand the criteria that determines when this is displayed. No doubt there is some criteria that results in its display. And why it's there as there does not seem to be any functional impact the average user would notice.

UPDATE:

As suggested in a comment, when the "?" appeared, I tried the command:
ping -c10 www.ubuntu.com

The result is below. A few minutes later when the wireless icon came back, I ran the command again, but could not see any meaningful difference.

PING www.ubuntu.com (185.125.190.29) 56(84) bytes of data.<br>
64 bytes from website-content-cache-3.canonical.com (185.125.190.29): icmp_seq=1 ttl=50 time=203 ms
64 bytes from website-content-cache-3.canonical.com (185.125.190.29): icmp_seq=2 ttl=50 time=201 ms
64 bytes from website-content-cache-3.canonical.com (185.125.190.29): icmp_seq=3 ttl=50 time=250 ms
64 bytes from website-content-cache-3.canonical.com (185.125.190.29): icmp_seq=4 ttl=50 time=171 ms
64 bytes from website-content-cache-3.canonical.com (185.125.190.29): icmp_seq=5 ttl=50 time=150 ms
64 bytes from website-content-cache-3.canonical.com (185.125.190.29): icmp_seq=6 ttl=50 time=216 ms
64 bytes from website-content-cache-3.canonical.com (185.125.190.29): icmp_seq=7 ttl=50 time=240 ms
64 bytes from website-content-cache-3.canonical.com (185.125.190.29): icmp_seq=8 ttl=50 time=161 ms<br>
64 bytes from website-content-cache-3.canonical.com (185.125.190.29): icmp_seq=9 ttl=50 time=183 ms
64 bytes from website-content-cache-3.canonical.com (185.125.190.29): icmp_seq=10 ttl=50 time=205 ms

www.ubuntu.com ping statistics
10 packets transmitted, 10 received, 0% packet loss, time 9009ms
rtt min/avg/max/mdev = 150.093/198.332/250.660/30.936 ms