Waited overnight and monitored the traffic in the firewall - not much activity at all from the Mac.
Googling around I found information about the cloudphotod and checked the logs and found the information below.
The error messages repeat every 25 seconds.
default 08:55:18.808572+0200 dasd 501:com.apple.cloudphotod.sync.discretionary-when-background:76874D:[
{name: ApplicationPolicy, policyWeight: 25.000, response: {Decision: Can Proceed, Score: 0.19}}
{name: DeviceActivityPolicy, policyWeight: 2.000, response: {Decision: Can Proceed, Score: 0.40}}
{name: MemoryPressurePolicy, policyWeight: 5.000, response: {Decision: Can Proceed, Score: 0.50, Rationale: [{[memoryPressure]: Required:2.00, Observed:1.00},]}}
] sumScores:45.934556, denominator:69.910000, FinalDecision: Can Proceed FinalScore: 0.657053}
error 08:55:19.318520+0200 cloudphotod Failed to fetch <private> in <private>: Error Domain=CKErrorDomain Code=1 UserInfo={NSUnderlyingError=0x7f8146434010 {Error Domain=CKInternalErrorDomain Code=5004 UserInfo={CKErrorDescription=<private>, NSLocalizedDescription=<private>, NSUnderlyingError=0x7f8146435400 {Error Domain=CKInternalErrorDomain Code=5004 UserInfo={CKErrorDescription=<private>, NSLocalizedDescription=<private>, NSUnderlyingError=0x7f81464336f0 {Error Domain=CKInternalErrorDomain Code=5000 UserInfo=0x7f8146435c20 (not displayed)}}}}}, CKErrorDescription=<private>, NSDebugDescription=<private>, NSLocalizedDescription=<private>}
error 08:55:19.319200+0200 cloudphotod Sync session in scope-update got error: Error Domain=CloudPhotoLibraryErrorDomain Code=80 UserInfo={CPLErrorRequestUUIDs=<private>, NSLocalizedDescription=<private>, NSUnderlyingError=0x7f814623c4c0 {Error Domain=CKErrorDomain Code=1 UserInfo={NSUnderlyingError=0x7f8146434010 {Error Domain=CKInternalErrorDomain Code=5004 UserInfo={CKErrorDescription=<private>, NSLocalizedDescription=<private>, NSUnderlyingError=0x7f8146435400 {Error Domain=CKInternalErrorDomain Code=5004 UserInfo=0x7f8146435d60 (not displayed)}}}, CKErrorDescription=<private>, NSDebugDescription=<private>, NSLocalizedDescription=<private>}}}, underlying error: Error Domain=CKErrorDomain Code=1 UserInfo={NSUnderlyingError=0x7f8146434010 {Error Domain=CKInternalErrorDomain Code=5004 UserInfo={CKErrorDescription=<private>, NSLocalizedDescription=<private>, NSUnderlyingError=0x7f8146435400 {Error Domain=CKInternalErrorDomain Code=5004 UserInfo={CKErrorDescription=<private>, NSLocalizedDescription=<private>, NSUnderlyingError=0x7f81464336f0 {Error Domain=CKInternalErrorDo
error 08:55:19.319469+0200 cloudphotod Sync session did fail with error: Error Domain=CloudPhotoLibraryErrorDomain Code=80 UserInfo={CPLErrorRequestUUIDs=<private>, NSLocalizedDescription=<private>, NSUnderlyingError=0x7f814623c4c0 {Error Domain=CKErrorDomain Code=1 UserInfo={NSUnderlyingError=0x7f8146434010 {Error Domain=CKInternalErrorDomain Code=5004 UserInfo={CKErrorDescription=<private>, NSLocalizedDescription=<private>, NSUnderlyingError=0x7f8146435400 {Error Domain=CKInternalErrorDomain Code=5004 UserInfo=0x7f8146435d60 (not displayed)}}}, CKErrorDescription=<private>, NSDebugDescription=<private>, NSLocalizedDescription=<private>}}}, underlying error: Error Domain=CKErrorDomain Code=1 UserInfo={NSUnderlyingError=0x7f8146434010 {Error Domain=CKInternalErrorDomain Code=5004 UserInfo={CKErrorDescription=<private>, NSLocalizedDescription=<private>, NSUnderlyingError=0x7f8146435400 {Error Domain=CKInternalErrorDomain Code=5004 UserInfo={CKErrorDescription=<private>, NSLocalizedDescription=<private>, NSUnderlyingError=0x7f81464336f0 {Error Domain=CKInternalErrorDomain Code=50
default 08:55:19.319578+0200 cloudphotod [session #91] failed during scope-update. Will schedule a restart from scope-update
default 08:55:19.319588+0200 cloudphotod <private> is done
default 08:55:19.319652+0200 cloudphotod Will back-off retry to 160s
