What @DraigFach said is wrong. Trustees are responsible for the proper running of the scheme, including paying benefits when they become due. They are required to act in the best interests of the scheme beneficiaries (this includes members and their next of kin/beneficiaries). They cannot just sit back and keep the funds because they can't be bothered to find a members beneficiary.
Depending on the type of scheme will depend how the benefits are paid out.
If it's a final salary scheme the rules will determine who is paid out. The rules will state financial dependents or legal spouses can receive a regular pension payment. There may also be a pension paid to children under the age of 23.
If there are no financial dependents or legal spouses there may be a one off lump sum paid. (Usually when the pension hasn't been in payment to the member or if it was paid for less than a specified number of years).
For regular pension payments there doesn't have to be a nomination, but there may have been a nomination for the lump sum payment.
If it is a pot of money type pension then the funds need to be paid out to someone, regardless of financial dependency (what I mean is you don't need to be financially dependent on the member to receive some or all of the pot). But if there is someone who is/was financially dependent on the member then they may be first in line for the pot to be paid to them.
Generally the scheme will start by looking at any nominations the member has made. Although they don't need to pay out to the nominees, they will generally try to follow the wishes. For example, if someone nominated their partner from years ago and has subsequently married someone else but forgot to change nomination then they may disregard the named beneficiary.
If there has been no nominations then they will look at who is next of kin, or who informed the scheme of the members death. There is usually someone they are in contact with to pay any funds due. For example,the scheme may ask the person who informed them of the members death if the member was married, had any children, living parents and or siblings. They will use this information to decide who to pay the funds to (they may ask this even if they had a nomination form).
There are of course rare occasions where there is absolutely no one (very rare) but in most cases there is someone to pay the funds to. The scheme is required to make an effort to find a beneficiary to pay funds to. They cannot just keep the funds for themselves.
Although the scheme does not have to follow what the Will states, they may use this to help decide who to pay out to (although most of the time they don't even look at the will)
The most common time for someone to make a nomination is when they first join the scheme, but they can amend their nomination at any stage.
So, this information probably hasn't given you any definite answers but could narrow it down if you think about the type of scheme they were a member of, what sort of payment you are getting/got (regular payments or a lump sum), if your relative would be likely to update nominations (some people just forget about pensions) and how you were related to them.
You can ask the scheme for copies of the members record which should include a nomination form if there was one (I'm not sure if they can tell you this but you can ask). You can also ask the scheme how they decided who the funds should be paid to.
I hope this helps.