You raise a very concerning question, because the houses that you speak of are incredibly dangerous. The processes used to synthesize methamphetamines are involved organic chemistry reactions that require extremely reactive ingredients. Some of those ingredients come in either solid, liquid, or gaseous forms, which can make their detection incredibly difficult- one can not tell what they are by looking at them, and one may not be able to see them or detect them by smell. Also, the chemicals are harmful in very different ways - some are violently reactive on contact with air (explosive), some are incredibly corrosive (cause excruciating, permanently damaging burns), and some cause terminal, chronic diseases or cancers.

I can list just a few examples of some of the chemicals that must be treated with extreme care that one might find left over in a house where methamphetamines were synthesized.

Phosphine gas - explosive
Metallic lithium or sodium - explosive on contact with air or water
Mercury - acute neurotoxic and carcinogen (causes sickness, death, or cancer over short or long times)
Ammonia liquid or gas - explosive, corrosive
Hydrogen chloride / muriatic acid - extremely corrosive - in liquid or gaseous form

Even if some of these chemicals are stored in a proper container, improper handling of the container can cause a violent (explosive) reaction.

Many of these chemicals are used in organic chemistry laboratories, although for very different purposes. There is great care taken in a laboratory to ensure the safety of those around, such as personal safety equipment (goggles, fire retardant clothing), engineering controls (fume hoods, blast shields, toxic gas sensors and alarms, etc), and regulatory controls (proper training on safety and handling, proper storage methods, etc). None of these are present in a house.

Because many of these chemicals are difficult to identify, it makes cleanup and disposal incredibly difficult and dangerous.