I have a 1995 Pontiac Bonneville SLE with original UIM.

My questions is as follows:
I live on Long Island and conditions are getting much colder here now in the past few weeks, especially at night. What i'm noticing is that the antifreeze level in the overflow tank drops MUCH lower overnight as it gets colder out (or seems to) but when the car is fully hot it'* still at or very close to "Full hot" where it was late summer when the car was fully warmed up. Tonight I was driving and it was rather cold, about 35 degrees. I was driving the car all the way on open highway for about 30 mins and of course the heat was on, so the temp gauge stayed a little lower than usual. When I pulled up to my house and shut the car off I immediately checked the anti-freeze level cause i always worry about a UIM failure. This time I noticed it about 2cm lower than "full hot" but I was figuring maybe cause it'* 35 out and I didn't really let the car get even hot enough for the cooling fans to come on cause of the weather. Can someone tell me if this is simply a matter of outside temperature, or am I looking at a possible UIM on the way? If it can be a UIM, can you tell me what other symptoms to check for to be certain, and about how fast these fail also from noticing these symptoms?

Thank you ahead of time to anyone who gives their advice/wisdom...it is highly appreciated.

have you checked your oil to make sure its not milky?

The oil seems of normal consistancy, I don't know exactly what to look for like i'm not a pro at this, but from all the oil i've checked in cars, mine seems very normal to me.

I noticed you said milky, could you maybe describe anything else I should be looking for in it, like I think I heard once bubbles or you can tell water is in it? If that is true then I don't have that issue.

if its milky you will know... its like the color of peanut butter

im gonna leave this in the hands of the gearheands now.. they will take care of ya

I don't think it'* the color of peanut butter and seems normal so I guess no UIM and I thank you for the advice, of couse anyone else wanting to chime in, please go ahead and do so.

when the car is cold, have you filled the coolant to where it is supposed to be when its cold, and then drove it till its hot, and then check to see where the coolant is?

Well, i'll explain it like this

When its colder at night like say 35 compared to 2 months ago at 65 or so, the antifreeze is well below "full cold" when i get in in the morning. Whereas in the summer it would be "full cold" level. Now, when the car heats up, regardless winter/summer, it still reaches "full hot" telling me there couldn't be a leak. The only thing that slightly worried me is tonight it was JUST under "full hot" but again it was 35 out and i drove pure highway with the heat on and then shut the car off right after, it never getting REALLY hot cause any other time i've checked its been after driving a while as it'* 50+ out

What you're experiencing is pretty much normal because outside temps are lowering with winter coming. The coolant level in the recovery tank can vary a little at the "full hot" reference line from summer to winter. However, if you notice a significant drop in level, this would warrant looking further into the coolant system or UIM/LIM.

That'* what I wanted to hear lol...thank you!

BTW if you ever want to know how to clean the inside of the recovery tank, I be the man. :o