Troubleshooting grounding problems can be difficult at best. It was not clear what you measured your voltage in reference to. In a floating DC system, if you read a voltage to ground, you have a ground in your circuit. If you do not, then your problem lies elsewhere (a problem with your system reference?). A cross connection to a different system can be a possible cause of this problem as can a bad power supply or a wiring mistake. Also, make sure that you do not have AC in your DC circuit

If you do have voltage to ground, you do have a ground in your power system but it is unlikely that a single ground would cause this problem (though not impossible) and a lot harder to find though there are ways to do it. When strange voltages appear in a system, it is commonly caused by a ground loop of some sort. One thing I would suggest is that you use the method Phil described at the closest you can get to your power supply upstream of any sensors and as much of your power distribution as you can and move outward toward your sensors to see if you can find ground loop current. This can eliminate the power distribution being involved and sometimes find the ground loop current path. Also note that the power supply does not necessarily have to be involved in the ground loop but can still be affected by it. If you still cannot find the ground loop, you can then try the sensors. Once you have found the first ground, you have done the easy part, finding the second ground can be more difficult and you should leave the first ground intact till you find the second ground. Also, be careful in disconnecting grounds as some times you do not know what they are connected to if there is another ground in the system.

It is also good to use your power system drawing or a system map to document as you go because these problems can be quite complex requiring some logic to figure out.