Did they return it with a full tank of gas too?
Good idea I guess but a lot of trouble IMO compared to a drop or two of food colouring.You could add a "bubbler" tube to your coolant reservoir to help you see the level.
Get a 1 foot piece of small diameter vinyl tubing. Drill a hole in the top of the reservoir slightly smaller than the outside diameter of the tubing.
Push the end of the tubing down to the bottom rear corner of the reservoir below the min and max marks.
When you want to check the coolant level blow a small quantity of air into the tube and watch the bubbles rise up through the coolant. You'll easily see the level.
You could also use this tube to add coolant as required although it would be a slow top off if you used a small ID tubing. But, you'll typically be adding a very small quantity of liquid so a top off could be quickly achieved.
Make sure the top end of the tubing is stowed above the coolant overflow hose connection. You don't want an overflow leaving via the tubing.
****!It's not okay. Just overheated in traffic.
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