Sarah,
OK, there is a lot to unpack here.
1st I wouldn't be too concerned about off gassing from the the sealant around the pipes. You should have enough airflow in the building to fairly quickly disperse off gassing from that small amount of sealant.
2nd Your inclination that blocking entry as your first step is spot on. Here Autumn is resulting in mice coming in to find shelter for the Winter so one additional thing to check is blocking holes near the ground line both above and below.
3rd Would not recommend steel wool in the heating venting system. If mice are getting in that system your facilities team has a bigger problem, potentially a compromised system that is losing efficiency. Better to find where they are getting in to the system and block entry. If you suspect they are coming through the heating system trap around the vents to confirm it. Rather than putting steel wool in the system which will affect efficiency try putting steel window screen on the inside of the vent covers.
4th It is interesting that you are seeing an increase of a long standing issue. Curious what the cause might be. If this is a building you share with others and therefore do not have control over policy in other parts of the building, that may be a factor. Again trapping will help determine where they are and possibly where they are coming from and you may be able to help facilities develop a plan to find out more information.
Good Luck!
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Bradford Bredehoft
CEO
Museum Study, LLC
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