Answer provided by Katrina Exter:

For chop-nod observations, a “Split On-Off” pipeline script in HIPE allows you to create separate off-source and on-source cubes, which you can check for contamination. For un-chopped range scan observations, the Level 2 of the on-source observation contains the non-background subtracted products, which can be compared to the Level 2 products of the off-source observation to look for contamination. For un-chopped line scans, on-source and off-source cubes are created at the end of the pipeline, however only the subtraction of the two is saved to Level 2 of the observation stored in the HSA, hence it is necessary to re-run the pipeline in HIPE to retrieve these cubes.

However, an Highly Processed Data Product called “On-source/Off-source cubes” has been created and the products are available through the HSA. These HPDPs contain separate on-source and off-source cubes for each observation, which can be opened in any cube viewer and inspected for contamination. The comprehensive Release Note accompanying these HPDPs includes a discussion of what to do if your off-source position was “contaminated”.

You can find the Release Note for the On-source/Off-source cubes, here:

http://www.cosmos.esa.int/documents/12133/996891/PACS+Spectrometer+HPDP+-+The+separated+On-source+and+Off-source+cubes