What would trackmarks look like after a death?

Probably when the body is recovered it would be frozen and maybe covered with frost. This could initially cover up tracks and bruising. Once the body thaws, the tracks would show up differently depending upon where on the body they are in relation to lividity. Assuming they are above the pooling of blood, you'd still see punctures in the skin and some pooled blood in the bruising. Pus from an infection would remain. Other than paler, not much different than when the person was alive.

Freezing would cause frost damage, but can be easily separated by any competent medical examiner.

Jim Clark-Dawe

Three days isn't long, especially in the cold. Infections don't heal upon death, so any discharges or wounds still show, they just tend to grow gray with the skin. As a heavy user, tracks would show up and signs would be noticeable at autopsy anyway. It's really hard to be a heavy user and not have a criminal and medical history showing that, which would be a big indication of what to look at as cause of death.

Jeff

I don't know what time period and location, but in most states, if not all, a body found under a bridge would require an autopsy. Unless the body disappears, it would be checked. Among other things, there is criminal liability for suppliers of illegal drugs for people who die of overdoses.

Basically, in most states, a body without a medically known cause of death is given an autopsy. A death at home with a terminal diagnosis or a logical medical diagnosis are the exceptions to the rule.

Jim Clark-Dawe

Image of tracks and a rash.

More tracks.

Add blackened skin for frostbite perimortem, pale greenish for decomposing, also blackened from decomposing, pale grayish typical, lividity on the side the person is laying on, and you might see frost on a cadaver laying out a couple days in freezing temperatures.

Postmortem Changes Warning, pics get worse as you scroll down.

You introduce a very imprecise factor here of her suffering frostbite, I'm presuming after death. There's a lot of difference between 35 degrees, 32 degrees and 29 degrees in how a body reacts. And assuming normal weather, you're talking a range from around 5 AM to 4 PM as the temperature rises and falls. At least under a bridge we don't have to worry about the sun warming the body. We might have morning frost to worry about, depending upon the moisture in the air.

The body will cool once the heart stops beating. However, there will be some heat generated by the continued digestion for a bit after death. But in a few hours, the body will be at the temperature of the air and the surface on which the body is lying.

In your case, this means that the body will be somewhere close to or below the freezing point. This will slow down the decomposition process to a virtual halt. And as WeaselFire points out, three days isn't much. Think about how little a pound of hamburg left in the refrigerator will change. If you throw the hamburg in the freezer, there will be essentially no change.

Your body will be extremely pale, to a greyish color, with the lower side showing marked lividity. Very like the first picture and not like any of the other pictures in the postmortem changes link. Other than the color, there would be no obvious decomposition at those sorts of temperatures. The frostbite damage would probably remain frozen and not have had a significant thaw cycle to cause the damage leading to blackened skin.

Damage shortly before death remains almost the same in this sort of scenario. Don't worry about the grimness. It won't exist. Now if you were talking about 90 degree temperature, high humidity and sitting in the sun for three days!!!

By the way, a lot of homeless have seen the dead. There's a reluctance to call the police in that community and reporting dead bodies is not high on their list of important activities.

Jim Clark-Dawe

It might be wise to craft a scene in which the homeless POV character discovers the body, sees the track marks, assumes it to be an overdose, and departs the immediate area. That behavior pattern is consistent with what usually really happens (involving the homeless). You can use subsequent scenes to describe dialogue between other (homeless?) characters in which the expository facts you feel need to be divulged regarding the decedent are brought to the readers' attention. This avoids getting into any forensic details that the POV character would not be privy to, beyond any previously described observation in the first scene.