Malware on Mikrotik output chain?

we have been informed by our ISP that there are Malware attached originated from our public IP.
Dst IP are only few so I’ve added them to a list and added firewall rule in forward, output and Mangle - Postrouting chain , trying to catch which LAN IP is doing this

/ip firewall filter add action=add-src-to-address-list address-list=MalwareLAN_IP address-list-timeout=none-dynamic chain=forward dst-address-list=Malware log=yes log-prefix=***Malware_LAN_IP_Forward**
/ip firewall mangle
add action=add-src-to-address-list address-list="Malware " address-list-timeout=none-dynamic chain=postrouting dst-address-list=Malware log=yes log-prefix=***Melware_Postrouting***

to my supprise, the only log I have got so far is on the output and postrouting chain,
log attached

09:13:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (SYN), myPublicIP:59285->13.248.148.254:80, len 60 
09:13:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (ACK), myPublicIP:59285->13.248.148.254:80, len 52 
09:13:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (ACK,PSH), myPublicIP:59285->13.248.148.254:80, len 456 
09:13:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (ACK), myPublicIP:59285->13.248.148.254:80, len 52 
09:13:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (ACK), myPublicIP:59285->13.248.148.254:80, len 52 
09:13:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (ACK), myPublicIP:59285->13.248.148.254:80, len 52 
09:13:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (ACK), myPublicIP:59285->13.248.148.254:80, len 52 
09:13:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (ACK,PSH), myPublicIP:59285->13.248.148.254:80, len 398 
09:13:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (ACK), myPublicIP:59285->13.248.148.254:80, len 52 
09:13:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (ACK), myPublicIP:59285->13.248.148.254:80, len 52 
09:13:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (ACK), myPublicIP:59285->13.248.148.254:80, len 52 
09:13:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (ACK), myPublicIP:59285->13.248.148.254:80, len 52 
09:13:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (ACK), myPublicIP:59285->13.248.148.254:80, len 52 
09:14:34 firewall,info ***MalwareIP output: in:(unknown 0) out:sfp-sfpplus1, proto TCP (ACK,FIN), myPublicIP:59285->13.248.148.254:80, len 52

I’ve checked, there is no script / scheduled task on Mikrotik that I don’t know of.
there are no proxy, socks service.
I don’t think the router is hacked.

only reason the Mikrotik originate a connection is Mikrotik’s own DDNS service in Clud, and a SNTP client

we used to have few VPN client using this Mikrotik as a VPN server but I would think anything from these VPN clients would show up in forward Chain?
any thoughts?

and netwatch have a up/down-scripts, vpn prifile script, dhcp script…
In System>Scripts you have a JOB tab, check it.
Use a Tool\Tourch too

and easier do /export file=dump.txt and analyze it.