A lot of factors would go into using thermal imagery for locating buried items. It would depend on the terrain first of all. If it is in an area that is wooded, it would be very difficult because you would not really be able to see the true surface under leaves, branches, or other debris. If it was recently buried, the disturbed ground and debris could be visible on the imager's screen.
If the area is something like a desert or a maintained lawn, slight indications could be visible for several years just from the surface settling, such as a crate rotting and collapsing, even just slightly. I have seen videos of air searches that clearly showed the outlines of graves in a cemetery that where decades old.
If there has been a recent drastic change in the temperatures over a short period, you may have an easier time locating buried objects because the buried item, iron, gold, etc, may hold the heat or cold for a longer time than the surrounding soil. This will be easier to detect from a higher view point like an aircraft or a drone. Of course, the deeper the item, the less temperature difference there will be. A good time to check an area would be early morning after the sun has heated the ground slightly, or late at night when the ground has cooled, but buried objects may still hold some daytime heat.
Although thermal cameras may seem simple to use, get some training and learn how different things appear when viewed in Infra-red.
I agree with MM, GPR will probably be a better choice, although much more costly. Again, GPR will also include a learning curve like any other technology.
I would be interested to hear how you make out, either way.