If I'm interpreting your intent correctly, you're looking for something that an initial investigator might overlook, or could be covered up by the initial investigator (with relative ease). Most chemistry labs will have acids, alkali's and solvents. In an of themselves, these may be caustic but not explosive, but in the right combinations and circumstances, certainly can form combustible, even explosive compounds. For example, sulfuric acid, sugar, and potassium chlorate will spontaneously ignite (see variations on Molotov Cocktail). What might indicate magic (or some outside influence, sabotage, etc) could be:

1. Where the source of energy (magic, fire, etc) originated. If there's no readily flammable material in that immediate vicinity, then the energy was thrown/cast/propelled from that spot into something flammable/combustible (storage cabinet, shelves filled with chemicals in beakers and flasks) causing the resulting chain reaction.
2. The type of damage is inconsistent with the materials present (magic doing damage which caused the explosion/fire, but the fire does not fully conceal the damage done by the magic)

No idea if any of this might help, or if I've even correctly determined what you're asking... so I'll stop right here.