Which clue would most likely indicate the presence of explosives indoors?

Loose electrical fittings can be a significant indicator of the presence of explosives indoors. When explosives are stored or used, they often require electrical components for detonation, such as wires or blasting caps. If there are loose electrical fittings, it could imply a recent alteration or tampering with electrical systems that might be related to illegal activities like the installation or preparation for using explosives. This clue is particularly relevant in a private investigation context, as it may suggest potentially dangerous situations that need immediate attention.

In contrast, the other clues such as recent soil disturbance, footprints in the garden, and tree marks in the yard do not provide a direct connection to the presence of explosives. Soil disturbance might indicate construction or landscaping, while footprints and tree marks are more associated with outdoor activities but do not signify anything about explosive materials or their use indoors. Thus, while each of those clues could lead to a variety of investigations, loose electrical fittings stand out as a more definitive sign that warrants further investigation concerning explosives.