  The fundamental vacuum state of quantum fields, related to Minkowski space,
produces divergent fluctuations that must be suppressed in order to bring
reality to the description of physical systems. As a consequence, negative
vacuum expectation values of classically positive-defined quantities can
appear. This has been addressed in the literature as subvacuum phenomenon. Here
it is investigated how a scalar charged test particle is affected by the vacuum
fluctuations of a massive scalar field in D+1 spacetime when the background
evolves from empty space to a thermal bath, and also when a perfectly
reflecting boundary is included. It is shown that when the particle is brought
into a thermal bath it gains an amount of energy by means of positive
dispersions of its velocity components. The magnitude of this effect is
dependent on the temperature and also on the field mass. However, when a
reflecting wall is inserted, dispersions can be positive or negative, showing
that subvacuum effect happens even in a finite temperature environment.
Furthermore, a remarkable result is that temperature can even improve negative
velocity fluctuations. The magnitude of the residual effects depends on the
switching interval of time the system takes to evolve between two states.
