They are attracted to light colored homes, usually older homes and they are attracted to heat that the homes reflect. Once ladybugs have penetrated the home though, they are hard to get rid of.
Ladybugs release pheromones. This helps ladybugs find each other and it lets future generations know of a good place to "camp out" for the winter. The pheromones don't go away easily. The chemical "scent" can remain year after year, and not only on the outside of a structure, but also within the walls, where ladybugs tend to hide before emerging into your home. So, scrubbing pheromones off a house is a BIG task, if not impossible.
The yellow stuff you might see from time to time is their blood. Releasing some of its blood is one way the ladybug can protect itself. The blood smells bad and signals to a predator that this ladybug is not a good lunch choice.
To prevent ladybugs from getting in, make sure all cracks around windows, doors, clap boards, pipes, etc. are sealed up. Some extermination companies offer this service, sometimes called inclusion.

Q. It is almost spring, why are ladybugs coming back into my house?
A. They have probably been hibernating under the sliding of the house or apartment and the warmer temperatures have caused them to emerge- it's just that they are going in the wrong direction.  You would think that they would be trying to get out of the house, but they are coming in. It happens. This happens because of the variation in temperatures from the interior of the home verses the outside temperatures. The ladybugs are merely confused.

Q. Are ladybugs poisonous?
A. No. Ladybugs are not poisonous to humans. However, they can have toxic effects on some animals. Ladybugs have a foul odor which deters some predators from eating them and their bright colors also help as a deterrent. In nature, red and orange, are warning colors that indicate to another animal or insect that the potential "lunch item" might not be a good choice.

Q. Why do ladybugs come into my house in the winter time?
A. Ladybugs are attracted to the light colored houses. Especially, homes that have a clear southwestern sun exposure. Older homes tend to experience more problem with aggregations due to lack of adequate insulation. The ladybugs come in through small cracks around windows, door ways and under clap boards. They want to hibernate in a warm, comfortable spot over the cold months of winter. Ladybugs gather in groups when they hibernate, so if you see one, you can be sure more will follow. The best way to keep them out is to repair damaged clap boards, window and door trim and to caulk small cracks.

Q. Once the ladybugs are in my house, will they eat anything?
A. No. Ladybugs don't eat fabric, plants, paper or any other household items. They like to eat APHIDS. Aphids are very small, but very destructive pest that feed on plants. (If you have rose bushes, you have probably seen aphids.) Ladybugs, while trying to hibernate in your house, live off of their own body fats. They, also, prefer a little humidity. But our homes are usually not very humid during the winter. In fact, they are rather dry causing most of your ladybug guests to die from dehydration. Occasionally, you might witness a ladybug in your bathroom getting a drink of water. Now, that's a smart lady!

Q. How can I get them out of my house?
A. If you don't have a lot, just leave them. They will leave when spring arrives. Disturbing them will only cause them to stress out leaving yellow markings on your walls. The yellow stuff, you see, is not waste matter, but rather, their blood. Ladybugs release a small amount of their blood which is yellow and smells, when they sense danger. Some people have said that it does stain on light colored surfaces.

Q. But, I really want the ladybugs out of my house!
A. Use a "shop vacuum". This type of vacuum is easy to use for collect ladybugs. When using this to vacuum up ladybugs, use a clean bag or pad the bottom with a cloth. After all is clean, release the unwelcome guests outside.

Q. Is there anything else I can use to get the ladybugs out of my house?
A. Yes. There is a product called a Ladybug Black Light Trap. It uses radiating black light to attract and contain the ladybugs. You will need to use a search engine to find a company that sells this product or something similar.