That symbol is just a place holder for a link/image/ or something else that cannot be accessed. Probably caused by the dock interfering with the link to the Internet or an image on your computer. Ignore it.

I have the same 'problem', because I see it as a warning for something.
If it is not a warning or I should ignore it, make sure that the yellow triangle is not there.
Adjust the software, and make sure it doesn't show up when Dell says I should ignore it.

Doing the Windows update has fixed my issue.

Same here, this started Friday but today I had Windows updates and Dell Support Assist suggested a critical BIOS update from last Wednesday - seems the yellow warning sign is fixed (for now)

I started seeing this issue this week after a Windows Update and a Dell BIOS update. Very frustrating that such "updates" cause problems like these!

I thinks it's because you are charging over the docking station (usb-c). USB-C (60W) doesn't deliver as much energy as the standard charger (130W). So your laptop charges / discharges at a lower speed.

This is caused by the incorrect power adapter being used.   To clear the exclamation mark,  reboot and enter the bios, go to the power event log and you will see the power event causing the exclamation mark.   Clear all log events and exit.  The exclamation mark will now be cleared!

Hope that helps

I actually reviewed any pending updates and found out that I had a few of them waiting to be installed, so I installed them, and after I reboot my machine, the yellow icon disappeared. 

This problem was caused (for me) by connecting another USB-C device (a monitor that required power) to the laptop. As I am powering the laptop via a hub, this additional power draw meant that the laptop 'thought' it wasn't getting sufficient power. Once I disconnected the monitor (and connected it to the hub instead) the warning triangle disappeared.