Probably is spiders. If you have not ever edited the agent table.
The default install only lists a few. So if you don't add a spider to the list they are a unknown visitor
There are a couple of posts here with personal settings.
Goto:
Control Panel » Display Options[+] Who's Online
Search Engine Agents
My list although not complete and probably never will is:
Accelovation=heritrix
Alexa=ia_archiver
AltaVista=Mercator
Altavista=Scooter
AllTheWeb=FAST-WebCrawler
Amazon=AMZNKAssocBot
Anzwers=AnzwersCrawl
AllTheWeb=crawler@fast
Ask=Ask Jeeves
Ask=teoma_agent1
Ask=Teoma
Atomz=Atomz
AskJeeves=AskJeeves
Babaloo=BabalooSpider
BecomeBot=BecomeBot
BoardReader=BoardReader
Boitho=boitho.com
Cityreview=Cityreview Robot
Cuil=Twiceler
CyberPatrol=CyberPatrol
del.icio.us Thumbnails=del.icio.us-thumbnails
DepSpid=DepSpid
DMOZ=Robozilla
DNS Mine=DNSMine
Dontbuylists=DBLBot
Dotbot=Dotbot
Entireweb=Speedy Spider
Excite=ArchitextSpider
Exalead=Exabot
Factbites=Factbot
Fast=FAST
Fast(AllTheWeb)=FAST-WebCrawler
Fast(AllTheWeb)=crawler@fast
Gigablast=Gigabot
Google=Googlebot
Google Mobile=Googlebot-Mobile
Google Images=Googlebot-Image
Google Adsense=Mediapartners-Google
Infoseek=InfoSeek sidewinder
InfoSeek=UltraSeek
Kalooga=Kalooga
libwww=libwww-perl
Looksmart=MantraAgent
LookSmart=FurlBot
Lycos=Lycos_Spider_(T-Rex)
MJ12bot=MJ12bot
Microsoft Research=MSRBOT
MSN Media=msnbot-media
MSN=msnbot
MSN Pic-Search=PSbot
Naver=Yeti/
NaverBot=NaverBot
NetSeer=Teemer
NorthernLight=Gulliver
noXtrum=noxtrumbot
Nutch=nutchsearch
Omgili=omgilibot
OpenAcoon=openacoon
Pagebull=Pagebull
Panscient=panscient.com
Picsearch=psbot
PrivacyFinder=PrivacyFinder
ScoutJet=ScoutJet
ScrubTheWeb=Scrubby
Search Me=Charlotte
Seznam=SeznamBot
Search Hippo=Fluffy the Spider
Shelob=shelob
Similarpages=Similar
SingingFish=asterias
Snapbot=Snapbot
SnapPreviewBot=SnapPreviewBot
Snoopy=Snoopy
Sogou=sogou web spider
StackRambler=StackRambler
StumbleUpon=StumbleUpon.com
TinEye=TinEye
TAMU=IRLbot
Teoma=TeomaAgent
TMCrawler=TMCrawler
Voila=Voila.fr
Voila=Voila
Voila=VoilaBot
Voyager=Voyager
Walhello=appie
WebAlta=WebAlta Crawler
Webcrawler=WebCrawler
Web Data Centre=WebDataCentreBot
Whois=SurveyBot
Wisenut=Zyborg
Worio=woriobot
Yahoo! China=Yahoo! Slurp China
Yahoo! Germany=Yahoo! DE Slurp
Yahoo!=Yahoo! Slurp
Yahoo! Blogs=Yahoo-blogs/v3.9
Yahoo! MM=Yahoo-MMcrawler
YaCy=yacybot
Yandex=Yandex
Yanga=Yanga
YodaoBot=YodaoBot
Yoono=yoofind
You can just cut and paste the list in your list
Probably will cut down on some of the unknowns anyway.
As new spiders come out some good some bad the list needs to be updated on a regular basis but it becomes a big task.
If you identify a bad spider then you can ban the ip.
