I always try to keep my detectors out of the direct sun and park in the shade if possible when it's hot. If it's above freezing I don't worry about the detector(s) and the cold, but colder than that I take them out as I wouldn't normally be using them anyway.
luvsdux
That's a good practice. A lot (I would venture to say MOST) of the problems surrounding electronics and weather extremes don't have as much to do with the temperature as it does the humidity. If the temp changes quickly and the air is humid, condensation can form on the detector. If humid air can get inside the control box, then quick temp changes can cause moisture to condense on the inside as well. This goes for ANYTHING; not just MD's.
Remember how a glass of ice water has so much condensation on a warm day? Moisture in the air hits the cooler glass and forms water. This happens because warm air can hold more moisture than cold air. WHICH, by the way, is why thunderstorms are far worse in the south than they are up where we are.

We have extremely humid air here and very often get temp swings, sometimes changing 50° to 60° in less than 20 minutes! Our house isn't the best insulated in the world, so sometimes things near an exterior wall will began forming a slight dampness. Mostly, this happens for us to things stored in non-climate-controlled areas, including closets in the house when the closet door is closed. Cardboard boxes will get damp - fortunately not enough to cause them to break down.
In any case, as warm air holds more moisture than cold air, I would recommend having your MD in a padded carry case when bringing it in from very cold temps, but only if you live in an area where the air stays humid. We're in wetlands, so our air is ALWAYS humid. Most people, however, probably won't have much problem most of the time. Basically, treat your MD the way a hunter would treat their rifle. Same rules apply. Let the rifle (or MD) acclimate to the temp of the house before exposing it to the warmer air. NOT doing this doesn't mean certain doom for your machine, but would be a good practice to get into. Temp and humidity is different in every circumstance, so just have to figure out for yourself what precautions should apply to you, if any at all.